The present invention relates to a novel insulating varnish and, in particular, it is concerned with an aqueous high molecular weight polyester insulating varnish which develops no harmful waste gases of organic solvents when it is applied onto a substrate and baked and is capable of forming a film possessing good electrical and mechanical properties. The term "aqueous" as used herein is intended to include the cases where the varnish is in an aqueous dispersion or an aqueous emulsion as well as in an aqueous solution.
In the part, there have been practiced methods of producing polyester synthetic resins which comprise reacting a dicarboxylic acid with a polyhydric alcohol in the presence of an organic solvent such as cresol and xylenol, and, if desired, adding to the reaction mixture a hardener such as lead octanoate an aliphatic or aromatic diisocyanate. Among these polyester synthetic resins the resins produced from an aromatic dicarboxylic acid as the starting material are excellent particularly in heat resistance and electric insulation and possess good solvent resistance and mechanical properties, so that they have been used as an insulating film for magnet wire and any other articles in various applications. Usually, in forming these films a solution of the polyester resin in an organic solvent is coated on the surface of a substrate article and baked whereby the organic solvent is vaporized. However, it is impossible to completely treat and recover waste gases of solvents developing during baking which are harmful to the human body. Therefore, there is a need for novel insulating varnishes which will not develop such harmful gases.
Recently, in the fields of, in particular, paint and lacquer productions a variety of wat r soluble or dispersible paints using water as a solvent have been developed instead of organic solvent type paints for reasons of safety and sanitation in handling. Thus, methods for the production of water soluble polyester resins have been developed which comprise adding a tricarboxylic monoanhydride such as trimellitic monoanhydride to water insoluble polymer containing primary hydroxyl groups such as a polyester and reacting the resulting mixture in the presence of an inert solvent for a long period of time, and converting the remaining free carboxyl groups to ammonium salts to produce an water soluble polyester resin.
However, the above-mentioned process includes the use of lower molecular weight polymer containing primary hydroxyl groups rather than higher molecular weight polymer containing primary hydroxyl groups because the latter provides no water soluble resin. Further, even if a derivative such as monoanhydride of a tricarboxylic acid is reacted with a lower molecular weight polymer, the derivative is bonded only to the terminal of the polymer molecule and cannot almost participate in polymerization, thereby producing no higher molecular weight resin. In this case, if the temperature is further raised to polymerize the resin to have a higher molecular weight, the resulting resin is entirely brought into a gel state. In addition, there is present another disadvantage of requiring further process for removing the organic solvents used in the reaction.
Further, in accordance with the above-mentioned process only polymers obtained from unsaturated carboxylic acids or alcohols having an unsaturated bond as the polymer containing primary hydroxyl groups may be made soluble in water, and the water soluble resin obtained from the method forms a film extremely poor in electrical properties and toughness when it is applied and baked. Thus, since it is requested that a useful varnish as an insulating film for magnet wires generally should be derived from aromatic materials and contain adequate high molecular weight resin and be capble of forming a film excellent in electrical and mechanical properties such as electric insulation, heat resistance, flexibility, toughness, etc., the above-mentioned process using tricarboxylic acids can never produce such a varnish that meets these requirements and utilizes water as a solvent.
There is known another method for the production of water soluble varnishes which comprises reacting a mixture of (a) trimellitic acid or pyromellitic acid, (b) allyl ether of an alcohol having at least one free hydroxyl group, (c) terephthalic acid and (d) a polyhydric alcohol till the acid value of the resutling resin is below 70, cooling the resuting resin and subsequently neutralizing the resin with ammonia or an amine. However, like the previous method, this method suffers from gellation at an early stage in the course of the reaction, which results in no formation of a high molecular weight resin solution, whereby a film having good electrical and mechanical properties cannot be produced.